Furnace construction for low temperature operation



March 1965 .1. w. THROCKMORTON ETAL 3,171,389

FURNACE CONSTRUCTION FOR LOW TEMPERATURE OPERATION Filed Sept. 5, 1963 Na 5 V w M M w m MM i N c 0 AT w w w 1 53 Z M a a J a .W II. in;

United States Patent 0 3,171,389 FURNACE CONSTRUOTION FOR LOWTEMPERATURE OPERATION John W. Throckmorton and John S. Wallis, New York,N.Y., assignors to Petro-Chem Development (30., Inc,

New York, N.Y., a corporation of Delaware Filed Sept. 5, 1963, Ser. No.306,726 1 Claim. (Cl. 122333) Large quantities of natural gas aretransported great distances by pipeline, and to take care of peak loadsat the point of use, such as for example the city of Boston, it has beenproposed to liquefy natural gas, largely composed of methane, at thepoint of production, for example near Beaumont, Texas, and to ship theliquid methane in specially insulated tanks which are then shipped bysteamer to Boston.

At this point of use the liquid methane will be stored at lowtemperatures, such as below 257 F. until the peak demand arises. It isthen necessary to convert the extremely cold liquid to vapor and gas bysupplying the necessary heat of vaporization from an external source.

If this heating is attempted to be done in the conventional tubular typeof furnace, the tube wall temperature on certain of the heating elementsis so low that moisture in the flue gas which is produced from theproducts of combustion forms a heavy coating of frost or ice.

The formation of frost or ice on the heating elements adversely affectsthe heat transfer rate and at the same time forms water in thecombustion chamber in the furnace. This causes damage to the furnace,reduces the flame temperature of the heater, and under severe conditionsmay cause the burner to be extinguished, making the furnace inoperative.

Other normally gaseous products such as propane and butane liquefy atcold temperatures which are higher than that required to liquefymethane, and in such cases the problem of icing or frosting is lesssevere.

One object of this invention is to provide a heater having tubularheating elements with means for insuring an adequate transfer of heat tothe tube walls so that either no ice or frost is formed on the tubes or,if some ice or frost is formed, the efliciency of the heater is not sub-\stantially reduced, because the heat transfer means has adequateheating surface spaced at a suflicient distance away from the tube wallsto insure this result.

Another object of this invention is to provide fin tubes in which thefins extend to an unusual extent beyond the surface of the walls, areL-shaped in cross section and have a comparatively large surface areasubstantially parallel to the surface of the tubes.

It should be understood that the use of fin tubes in the radiant sectionof furnaces is new and is advantageous only for the special purposesabove indicated. If the furnace were used for heating oil or water tohigh temperatures, such as for example 800 F. to 1200" F., the finsexposed to the radiant heat would be destroyed.

Other objects and advantages of the invention will appear from thefollowing description and attached drawings, in which:

FIG. 1 is an elevation of a vertical tube heater in which the primaryheating elements are formed to accomplish the purposes above set forth.

FIG. 2 is an enlarged sectional elevation of a portion of a fin tubeconstructed totransfer heat inwardly from the outer surfaces of the finto the walls of the tube on which the fins are formed. In this figurethe fins are mounted on the tubes in the form of a helix.

FIG. 3 is a cross-sectional elevation on a large scale of a heating tubehaving fins similar in formation to those shown in FIG. 2, but arrangedlongitudinally of the tubes.

" 3,171,38 9 c6 Patented Mar. 2, 1965 FIG. 4 is a cross-sectionalelevation on a large scale of a tube having fins of similar crosssection but attached to the tube in the form of rings.

Referring specifically to FIG. 1, a well known type of vertical heateris here shown composed of a vertical cylindrical furnace chamber 10having a concrete bottom 11 in which one or more upshot burners 12 arelocated and a single row of vertical tubes 13 which are spaced a shortdistance inwardly from the walls of the furnace chamber 10 and arelocated in a circular pattern. The tubes are interconnected in a seriesor parallel arrangement as desired by the use of the usual return bends14 at the top and bottom of the tubes. A vertical extension 15 of thefurnace chamber is smaller in diameter than the furnace chamber 14) andhas mounted within it a coil 16 which is connected to the main furnacecoil made up of tubes 13 by external connector 20.

The usual stack 21 is supported at the top of the convection section ofthe furnace and extends upwardly therefrom.

The hot flame from the burners 12 projects upwardly through the centerof the furnace and largely gives up its heat to the fluid in the tubes13 by radiation. The hot gases from the top of the main furnace thenbathe the convection coil 16 in the usual manner and pass upwardly outof the stack 21.

To carry out the intended operation, very cold liquid such as liquefiedmethane is introduced into the coil composed of tubes 13 at the bottom,as indicated by arrow 25, and the vapors and gases resulting from theheating of the liquid escape through the outlet, as indicated by arrow26.

According to FIG. 2, each of the tubes 13 has a specially constructedfin structure which, as here shown, is composed of a channel 30 having aweb section 3-1, a hub section 32 and a flange or parallel extension 33at the outer end of the web. This channel is wound onto the tube 13 in ahelical form with the hub members 32 welded or otherwise securelyfastened to the outer surface of tube 13. The turns of the helix aresufficiently close that the flange extensions 33 are close together andform a protective surface around the tube 13. This is clearly indicatedin the cross section of FIG. 2.

When very cold liquid such as liquefied methane is introduced into thetubes 13, the radiant heat from the furnace is primarily applied to theflange extensions 33 of the fins. The fins are formed of suitable metalso that the heat is immediately conducted through the fins to the tubes13, and if frost or ice forms on the exterior surface of the tubes 13,it will not interfere with the continuing transfer of heat from the finsurfaces 33 to the tubes by conduction and to the liquefied methanewithin.

In this way the liquefied methane is efi'ectively vaporized and thenecessary heat of vaporation, which is very large, is adequatelysupplied from the furnace without danger of burning the tubes.

Referring to FIG. 3, an alternate structure is here shown in which thetube 13 is shown in transverse section and the fins, of substantiallythe same shape as already described with reference to FIG. 2, areapplied. In this case, however, the fin structure is appliedlongitudinally of the tubes 13, but the flange portions 33 of one finextend close to the flange portion of the next fin and thus effectivelytransfer heat from the furnace gases to the tubes 13 regardless ofwhether there has been frost 0r icing formed on the exterior surface ofthe tubes.

Another alternative is shown in FIG. 4, in which there are mounted onthe tube 13 a series of rings which are close together and are Welded orotherwise attached to the tube by the hub sections 32. The rings incross sec- 3 V 7 tion correspond to those of the'fins of either FIG. 2'or FIG. 3. l

It is well known that the quantity of heat'required for vaporization ofliquefied gases is high and is usually referred to as the heat ofvaporization, For this reason the use of. fin tubes in a radiant sectionof the furnace, where temperatures are extremely intense, is feasibleand ,serves to improveithe transfer of heat from the radiant flames andhot combustion gases in the center of the heater to the liquefied'gasesWithin the tubes.

Fin tubes are,of course, well known in furnace structures, but the finsare usually formed primarily to increase to a slight or moderate extentthe exterior surface of the tube and are normally employed in theconvection section of the heater where the tubesare bathed in the hotgases ofthe furnace. On the other hand, according to this invention thefins are applied to the tubes which are subjected to'radiant heat andare arranged and designed in order that they may be peculiarly adaptedfor their intended purpose, as above described. 7

Other modifications of the shape of the fins may be made withoutdeparting from the spirit of the invention,

and only such limitations should be imposed as are indicated in theappended claim; 4

We claim:

methane or other low molecular weight hydrocarbons,

which comprises a plurality of tubes substantially spaced from oneanother and arranged in a tall, hollow, vertical [2 5 A heater forgasifying cold liquids such as liquefied members and the flange members.

References Cited by the Examiner UNITED STATES PATENTS 2,234,423 3/41Whittmann 122-367 X 2,419,233 4/47 spender 165 182 X 2,994,307 a 8/61Throckmorton etal. 1-22356 V FOREIGN PATENTS 584,192 9/59 Canada;735,384 8/55 Great Britain.

' 7 OTHER REFERENCES I 7 Cryogenic Fluids'Heated With No Ice on Tubes,published in Chemical Engineering periodical, Apr. 16, 1962, vol. 69,No. 8, page 104. V

'PERCY/LLPATRIVCK, Primary Examiner.

KENNETHW. SPRAGUE, ROBERT A. OLEARY,

' Examiners.

